This invention relates to a central security system for a building and, more particularly, to circuitry for selecting audible warning signals and for monitoring such signals, as well as other command and communication signals employed in the security system.
There is presently a growing requirement for providing large buildings with systems which can detect emergency conditions. For example, in large apartment or office buildings, smoke detectors and the like may be located throughout the building with each detector being connected to a central monitoring console which is to be manned at all times. One such system is shown in the U.S. Pat. No. 4,342,985. The system employs circuitry for addressing detectors in various parts of the building (or other facility) which is to be protected. The addressing of remote areas is synchronized with the addressing of electrical equipment in a central station to permit the use of time-division multiplexing and the transmission of signals from the detectors located in various remote sites. The multiplexed signals are communicated by a single monitoring wire to a central console for illumination of status lamps on the console.
While the addressing of signals in the foregoing patent is illustrated with switch arrays and AND gates, for both monitoring and control functions, it is to be understood that such addressing and multiplexing can be accomplished with other forms of addressing units such as those employing counters and read-only memories which drive electronic selector switches or multiplexers. Also, while the encoding of data is illustrated therein by the changing of bits in a serial two-bit signal, it is to be understood that the system may employ a more complex form of digital encoding of the data provided by the various detectors and sensors. In addition, the distribution of clock signals from a central clock to the outlying areas could be replaced with a set of satellite clocks synchronized with a common synchronization signal.
A problem arises in that the foregoing detector and sensing circuits are restricted to a sensing of environmental conditions such as heat, smoke and door openings. However, there are other functions relating to the transmission of communications, such as the continuity of circuitry for announcements, telephone lines, digital control lines, and sound generation (whoops and chimes) circuits which should also be monitored. In addition, it is desirable that the sound generation circuitry, as well as the circuitry for monitoring communications, should have a format of construction which allows easy replacement and substitution into a variety of security systems.